1. Field of the Invention
The present invention relates to a carbon fiber web which includes polymer nanofibers to achieve improved characteristics, thus being suitable for use in a gas diffusion layer for a fuel cell.
2. Description of the Related Art
A key component of a fuel cell is a membrane electrode assembly (MEA) which includes an electrode catalyst, a polymer electrolyte membrane, and a gas diffusion layer (GDL) integrated with each other. The performance of the fuel cell is determined by the performance of the elements constituting the membrane electrode assembly. Particularly, the gas diffusion layer is a very important material that simultaneously performs many functions, such as supporting the catalyst, conducting electricity generated from the electrode catalyst, uniformly dispersing and supplying reaction gases, and discharging water as a reaction product.
The gas diffusion layer is typically produced by impregnating an about 200 μm thick carbon fiber web with a resin and a conductive graphite powder, curing and carbonizing the impregnated carbon fiber web to prepare a gas diffusion medium (GDM), and forming a water repellent coating and a mesoporous layer (MPL) composed of a catalyst-supported carbon powder on the gas diffusion medium. That is, the carbon fiber-made gas diffusion medium and the catalyst-supported porous catalytic layer (CL) constitute the gas diffusion layer.
It is more advantageous for the gas diffusion layer to have 1) lower electrical resistance, 2) smaller thickness reduction at a stack assembly pressure (about 10 bar), 3) higher tensile and flexural strength, 4) higher gas permeability, and 5) lower water wettability. Accordingly, the characteristics of the gas diffusion layer are closely correlated with the characteristics of the gas diffusion medium, which is a material for the gas diffusion layer. That is, improved characteristics of the gas diffusion medium are directly connected to an improvement in the performance of the fuel cell. Therefore, improved characteristics and low production cost of the gas diffusion medium are very important considerations in improving the technology and price competitiveness of the fuel cell.
Water repellent carbon fiber webs with high gas permeability and electronic conductivity have been generally employed in gas diffusion layers for fuel cells. Such carbon fiber webs are produced by numerous methods. For example, a carbon fiber web is produced by a method based on a wet-laid process. According to this method, carbon fibers are dispersed in an aqueous solution, and then the dispersion is dehydrated, formed into a sheet, and dried using a wet paper making machine to produce a carbon fiber web. Particularly, since the carbon fiber web should undergo a series of continuous processes, such as resin impregnation, curing, carbonization and coating, for the production of a gas diffusion medium, it is required to have improved physical properties, such as high tensile strength and flexural strength. Improvements of raw materials for the carbon fiber web are also needed to decrease the thickness reduction rate of the carbon fiber web at a fuel cell stack assembly pressure and to reduce the electrical resistance of a gas diffusion layer. Conventional methods for producing carbon fiber webs require the addition of binders other than carbon fibers. However, a disadvantage of the carbon fiber webs is insufficient tensile strength due to weak bonding between the carbon fibers.